This invention relates to a multistage hydraulic machine, and more particularly to a multistage hydraulic machine to be operated in a pumping mode of operation or in a synchronous condenser mode of operation.
In general, a hydraulic turbine or a pump-turbine coupled directly to a synchronous generator is required to perform an in-air operation such that adjustable wicket gates and inlet valves are fully closed so as to pressurize the water by means of compressed air thereby to depress the water around runners. This occurs either when a synchronous condensor mode of operation is carried out in order to compensate for a decline in power factor in a power system, or when a pumping mode of operation is carried out during start-up of a pump operation. In a multistage hydraulic machine including plural pressure stages varying from a high pressure stage to a lowest pressure stage which are interconnected by so-called return passages and with a set of adjustable wicket gates respectively provided with outwardly arranged runners within the respective stage chambers, when such a water pressurizing operation is carried out, it is essentially required to perform smoothly a return operation from a water pressurizing operation to a turbine-pump operation. To that end, air exhausting passages are provided opening toward an inner surface side as possible with respect to the respective water flow passages of runners within the respective stage chambers.
Referring FIG. 1, an example of a two-stage pump-turbine of a conventional multistage hydraulic machine will be described. FIG. 1 shows a state upon the return from a water pressurizing operation to a normal operation such as a turbine operation or a pump operation. In FIG. 1, reference numeral 1 designates a main shaft mounting two runners such as a high pressure stage runner 2 and a low pressure stage runner 3. A spiral casing 4 is provided outwardly from the high pressure stage runner 2. A set of adjustable wicket gates 5 of a high pressure stage are provided in a flow passage between the spiral casing 4 and the high pressure stage runner 2. Further, a top cover 7 is provided through a specified space 6 to a back portion of the high pressure stage runner 2. A top cover side runner seal 9 and a lower cover side runner seal 10 are provided respectively between the top cover 7 and the high pressure stage runner 2, and also between a lower cover 8 provided downward of the high pressure stage runner 2 and the high pressure stage runner 2 for the purpose of sealing pressurized water that flows thereinto from the water flow passage so as to lower the pressure thereof. A main shaft sealing device 11 is provided between the main shaft outer surface near a portion on which the highest pressure stage runner 2 is mounted and the top cover inner surface opposing the main shaft outer surface in order to prevent pressurized water within the back portion space 6 from leaking out through the outer surface of the main shaft 1. A communicating passage 12 is provided in the high pressure stage runner 2 by which the water flow passage of the high pressure stage runner 2 communicates with the back portion space 6. An air exhausting pipe 13 is provided with the top cover 7 and has one end opening in the back portion space 6 and its other end extending to the external portion. A control valve (not shown) is provided with the air exhausting pipe 13 and opens upon the return from a water pressurizing operation to a normal operation so as to form an air exhausting passage that exhausts compressed air within the high pressure stage runner chamber.
The low pressure stage runner chamber and the high pressure stage runner chamber are interconnected by a return passage 14, and a set of adjustable wicket gates 15 are provided in the return passage 14 outwardly of the low pressure stage runner 3. Between the low pressure stage runner and a fixed portion, there are formed runner seals which are similar to those of the high pressure stage. However, a communicating passage 16 is provided inside the main shaft 1 extending from a water flow passage of the low pressure stage runner 3 to an external portion. On the extension of the communicating passage 16, there is connected an air exhausting pipe (not shown) coupled to a control valve which is to be opened upon the return from a water pressurizing operation to a normal operation, thus forming an air exhausting passage that exhausts the compressed air within the low pressure stage runner chamber. Further, a draft tube 17 is provided downward of the low pressure stage runner 3 communicating to a tailrace (not shown).
In the conventional multistage hydraulic machine with the aforementioned structure, as the compressed air is exhausted to perform the return from a water pressurizing operation to a normal operation, the water surface is raised within the respective stage runners, and since the respective stage runners mounted on the main shaft 1 are rotated, water that has reached the respective runners is repelled toward an outer circumferential side of the respective stage runner chambers, from which water is becoming filled. In this case, exhaust passages provided with the respective stages are opened in the inner circumferential side of the water flow passages of the respective stage runners, so that the compressed air does not remain in the respective stage runner chambers but is exhausted through exhaust passages toward the external portion in directions shown by the arrows in FIG. 1. This can achieve a return operation from a water pressurizing depressing operation to a normal operation with no problem.
However, after an operation mode has once returned to a normal operation, since the water flow passage of the high pressure stage runner 2 and the runner back portion space 6 are communicated by the communicating passage 6, with respect to the runner back portion space 6, there is exerted an outlet pressure of the high pressure stage runner 2, that is, a pressure which as high as half the total head when operated as a turbine or a pump. Therefore, the main shaft sealing device 11 must seal such high pressure water filled within the runner back portion space 6, and is inevitably exposed to an extremely harsh operational condition. As a result, there is experienced severe wear of packings that constitute the main shaft sealing device 11, and in the worst case there is a danger of packing-burning and damage of the main shaft 1. So this has been a serious problem.
To alleviate the water pressure exerted on the main shaft sealing device 11, countermeasures even while in normal operation have been considered, such as opening the control valve provided with the air exhausting pipe 13 so as to drain the pressurized water of the runner back portion space 6 through the air exhausting pipe 13, or especially providing a water draining pipe (not shown) that has an opening in the runner back portion space 6 and communicates with an external portion so as drain pressurized water of the runner back portion space 6. However, even with such countermeasures, there still remains the problem that in order to alleviate the water pressure exerted on the main shaft sealing device 11, a considerably large amount of water need be drained, and this causes a decline in the efficiency of turbine operation and pump operation. The aforementioned problem becomes more remarkable as the total head is increased.